1. Technical Field
This application relates to a device for use in orthopedic spine surgery. In particular, the present invention relates to an artificial disc replacement device that replaces a damaged or diseased intervertebral disc.
2. Background of Related Art
The human spine is composed of thirty-three vertebrae at birth and twenty-four as a mature adult. Between each pair of vertebrae is an intervertebral disc, which maintains the space between adjacent vertebrae and acts as a cushion under compressive, bending, and rotational loads and motions. A healthy intervertebral disc has a great deal of water in the nucleus pulposus, which is the center portion of the disc. The water content gives the nucleus a spongy quality and allows it to absorb spinal stress. Excessive pressure or injuries to the nucleus can cause injury to the annulus, which is the outer ring that holds the disc together. Generally, the annulus is the first portion of the disc that experiences injury. These injuries are typically in the form of small tears. These tears heal by scar tissue. The scar tissue is not as strong as normal annulus tissue. Over time, as more scar tissue forms, the annulus becomes weaker. Eventually this can lead to damage of the nucleus pulposus. The nucleus begins to lose its water content due to the damage; it begins to dry up. Because of water loss, the discs lose some of their ability to act as a cushion. This can lead to even more stress on the annulus and still more tears as the cycle repeats. As the nucleus loses its water content, it collapses, allowing the vertebrae above and below the disc space to move closer to one another. This results in a narrowing of the disc space between the two vertebrae. As this shift occurs, the facet joints located at the back of the spine are forced to shift. This shift changes the way the facet joints work together and can cause problems in the facet joints as well.
When a disc or vertebrae is damaged due to disease or injury, standard practice is to remove all or part of the intervertebral disc, insert a natural or artificial disc spacer, and construct an artificial structure to hold the affected vertebrae in place to achieve a spinal fusion. In doing so, while the diseased or injured anatomy is addressed and the accompanying pain is significantly reduced, the natural biomechanics of the spine are affected in a unique and unpredictable way. More often than not, the patient will develop complicating spinal issues in the future.
To that end, there is an overall need to treat the disease or injury while maintaining or preserving the natural spine biomechanics. Normal spine anatomy, specifically intervertebral disc anatomy, allows one vertebra to rotate with respect to its adjacent vertebra about all three axes. Similarly, the intervertebral disc also allows adjacent vertebra to translate along all three axes with respect to one another.
For the above stated reasons, a need exits for an implantable device which may be used as an artificial disc replacement thereby maintaining disc height and motion. More specifically, the motion maintained must address at least the principle motions of rotation about all three axes. The device must also have a means to inhibit or minimize expulsion of the device from its installed location. The implantable device has an additional need of having a prolonged life span in the body that can withstand early implantation, as is often indicated for younger patients. In addition, the implantable device will have a limited amount of particulate debris so as to reduce complications over the useful life of the device.